Mesquite trees displacing southwestern grasslands

Mesquite trees and woody shrubs are better adapted than grasslands to a
Southwestern climate predicted to shift toward higher temperatures and
greater variability in rainfall, UA ecologists have discovered.

As the desert Southwest becomes hotter and drier, semi-arid grasslands
are slowly being replaced by a landscape dominated by mesquite trees,
such as Prosopis velutina, and other woody shrubs, a team of University of Arizona researchers has found.

In a "leaf-to-landscape" approach, the team combined physiological
experiments on individual plants and measurements across entire
ecosystems to quantify how well grasslands, compared to mesquite trees
and woody shrubs, cope with heat and water stress across seasonal
precipitation periods.

"Our results show that even the smallest mesquites are better adapted
for thriving under elevated temperatures and dry conditions – the
projections for our future climate – suggesting that these woody plants
are here to stay," said Greg Barron-Gafford, an associate research
professor with the UA's Biosphere 2 who led the study, which is
published in the scientific journal Global Change Biology.

Over the last century, the face of the Southwest has changed. Before
heavy cattle grazing, flowing carpets of grass comprised of, for
example, Muhly (Muhlenbergia porter) or Grama grass (Bouteloua eriopoda) blanketed much of Southern Arizona's open range. Today, woody plants like mesquite trees dominate the landscape.

Native to the region, mesquites have been here for a long time, but not
in today's abundance, Barron-Gafford said. "Visitors that see our
experimental display at Biosphere 2 are always surprised to hear that
the Sonoran Desert in this area used to look very different a century
ago."

Scientists have evidence to believe woody plants began displacing
grasslands as a result of overgrazing, but has since been propelled by
changing climate.

"If there are too many cattle, they have the same effect as a lawn
mower," Barron-Gafford said. "They're tilling the soil, and because they
don't eat the prickly things, they stay away from the established
mesquite trees. But they consume their pods and drop them off in little
fertilizer islands. It's a perfect formula for landscape change."

Mesquite trees, the research team discovered, benefit not only from a
changing landscape, but also from a climate shifting toward higher
temperatures and greater variability in rainfall.

This was surprising, given that, evolutionarily speaking, grasses are
better adapted to hot and dry conditions because they use a modified
biochemical pathway for photosynthesis, the process by which plants take
in carbon dioxide from the atmosphere and convert it into sugars.

"Plants have to open pores in their leaves to breathe in carbon dioxide,
and while their pores are open, water diffuses out," Barron-Gafford
explained. "Compared to grasses, mesquite trees have to keep their pores
open longer for the same amount of sugars they make, meaning they lose
more water in the process."

But mesquites overcome their physiological disadvantage with roots that
reach down to 160 feet or more; they can tap into groundwater not
accessible to plants with shallow root systems.

"Mesquites waste more water, but they can access it much better,"
Barron-Gafford said. "Their roots are always out there and they find it,
allowing them to bypass the grasses' evolutionary advantage. These
deep-rooting shrubs and trees are accessing deeper water that was
previously unavailable to drive plant biology in this area."

"It levels the playing field," he said. "In the pre-monsoon season in
April and May, when the land is very dry and grasses are browning, the
mesquites are leafing out. You could say they have their toes dangling
in the groundwater pool. In an athletic analogy, it's like some
sprinters are already running, while the competition is still getting
their shoes on. "

"All of the benefits we associate with these shrubs, such as potentially
greater carbon capture from our atmosphere, increased shade, attraction
of wild life, and so forth come at the cost of greater water use," he
said.

The research aims to improve the models used by climatologists to
predict future climate. Current models, said Barron-Gafford, have mostly
relied on plant physiology alone. "In an age that will be defined by
our management of water resources, it is important to understand the
role that this woody plant encroachment has on our regional
ecohydrology."

"The known differences in photosynthetic biochemistry have led the
scientific community to expect that grasses should be better adapted to a
warming climate because they have evolved to be more efficient with
less water in higher temperatures and drier conditions."

The new findings reveal that a plant's structure – such as deep roots –
plays a more important role in how vegetation adapts to a changing
environment than leaf physiology.

His research team used a "portable biosphere" to test how plants perform
under different environmental conditions and temperatures. The
shoebox-sized device can accommodate a leaf or a branch of a living
plant and allows the researchers to seal it off from the environment.

The team made its measurements shortly before, during and after the
monsoon in the San Pedro Riparian National Conservation Area in
Southeastern Arizona.

"With our portable biosphere we can obtain snapshots of plant activity,"
Barron-Gafford said. "The device allows us to set and control
parameters like temperature, light and relative humidity while we
measure the carbon dioxide going in and out of the leaf."

He explained that the dry periods before and after the monsoon are
different from each other because once the monsoon rains cease, plant
metabolism is still running on high.
"Everything is cranking, except the remaining rain water is now deep
underground, and we are measuring at a time when the shallow-rooted
plants no longer have that pool to dangle their toes in. That is when
the stress comes back for them."

Mesquite trees, on the other hand, were found to be capable of running
their photosynthetic metabolism at half their maximum rate regardless of
whether it was hot and dry.

"Grasses can only function in a very narrow temperature range,"
Barron-Gafford said. "While the they are drying out, it's like a hot day
by the pool for the mesquite."

"A heavy monsoon downpour saturates the soil, but the surface dries off
fast because it's a hot and dry environment," Barron-Gafford said. "But
deeper down, the water remains in the soil longer, where the mesquite
can access it, but not the grasses. That is a really important
implication because our predicted precipitation patterns over the next
century are going to favor deeper rooted mesquite shrubs over
shallow-rooted grasses."

Because they rely much less on rainfall, mesquites should benefit from
the forecasted increasing precipitation variability, offering a possible
explanation for the observed encroachment of mesquite plants even in
areas no longer subjected to heavy grazing.

To monitor the interactions among plants, water, the soil and the
atmosphere on an ecosystem-wide scale, researchers with Biosphere 2 and
The U.S. Department of Agriculture operate so-called eddy covariance
towers in areas such as the San Pedro River Riparian Area, the Santa
Rita Mountains and the Catalina Mountains, which can measure
temperature, moisture and gases at defined heights above ground.

"With the towers, we can measure carbon, water and energy flow across an entire ecosystem," Barron-Gafford said.

In addition, his team has transplanted grasses and mesquite saplings
into large, highly instrumented drums within Biosphere 2, where the
researchers can put the plants through intentional periods of drought
and temperature stress. Visitors to Biosphere 2 can see this science in
action, right along the tour route, and learn more from members of the
research team as they are doing the experiments.

"In the outside world, we actively monitor these leaf and ecosystem
level responses to environmental stress, but to truly test the spectrum
of responses, sometimes you need more control than nature affords you,"
Barron-Gafford said.

He said that encroachment of woody plants onto former grasslands exposes
the area's semi-arid landscape to a higher risk of irreversible
desertification.

"If a fire runs through and burns the thick grass cover that stabilizes
the soil, you end up with areas consisting mostly of dirt with mesquite
and creosote bushes. The big dust storms in certain parts of Arizona are
visual proof how vulnerable that type of landscape is to erosion."

Filling gaps in knowledge about what climate predictions mean for plants
and animals in the region and making them accessible to land management
officials is an important goal of Barron-Gafford's research. "Our great
historical knowledge is not enough given future climate change
scenarios and we need biological data in addition to climate data to
make accurate predictions."